Bug 31041: (follow-up) Clean up and generalise

Bug 31041: (follow-up) Clean up and generalise

This patch cleans out the print media css rules that were originally
added for modalprinting.  The window.open method is simpler to maintain
and will more reliably print modal content as expected.

We factor out the printer code into it's own JS asset that we can apply
to other printable modals and then use it in the two existing places
where such modals have been defined already (cashup summary and bundle
confirmation).

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>